1) Field of the Invention
The present invention relates to an image forming apparatus that forms color images, an alignment pattern detecting sensor for the image forming apparatus, a method of determining an acceptance width for the alignment pattern detecting sensor, and a method of forming an alignment pattern.
2) Description of the Related Art
Conventionally, a color image forming apparatus that has a photosensitive drum and a revolver type developing device takes a main stream in the market. This type of color image forming apparatus forms toner images of respective colors, superposes the toner images on an intermediate transfer element to make a combined color image, and batch-transfers the combined color image to a recording medium.
On the other hand, with a recent demand of high speed and high performance for the color image forming apparatus, a four-drum tandem type color image forming apparatus has become popular. The tandem type color image forming apparatus has a configuration that a plurality of image forming units each including a pair of a photosensitive element (image carrier) and a developing device are disposed for each color at positions near a transfer belt, and toner images on the image carriers are sequentially transferred to a recording medium or a transfer belt to form the combined color image.
With this type of color image forming apparatus, since the toner images formed on the image carriers for respective colors can be transferred virtually at the same time, the tandem type color image forming apparatus has an advantage in that the printing speed can be increased. On the other hand, there is a disadvantage with respect to color misalignment between the respective colors, as compared to the conventional one-drum intermediate transfer type color image forming apparatus.
With regard to the technological problem of the color misalignment, many correction methods have been proposed. For example, Japanese Patent Kokoku No. Hei 7-19084 discloses a technology of correcting the color misalignment by forming line images for respective colors on a transfer belt, detecting passing of the line images by a detecting sensor, and measuring each offset from an ideal passing timing of the line images.
Since such a technology is a method of detecting an edge of a pattern passing through the detecting sensor, the detection accuracy is determined by the sampling frequency. In other words, if a machine has a resolution of 600 dpi and the correction unit is 42.3 micrometers (=25.4/600×1000), at least a detection of ±½ of the correction unit (=21.7 micrometers) is required. When the linear velocity of the line image on the transfer belt is 125 mm/sec, a minimum required sampling frequency is calculated as at least 6 kilohertz, from an equation of [sampling frequency]=[linear velocity]/[25.4/resolution dpi/2], but the detection accuracy (=detection error) in this case (=6 kHz) becomes 21.7 micrometers.
If this calculation is directly fed back to the misalignment correction, there may be no problem with such a degree of sampling frequency. However, there is a case where it is necessary to use this detection result (=x micrometers) for other operation. For example, when such detection is performed at left and right opposite ends of a recording medium with respect to the recording medium conveying direction and skew correction is performed or magnification error correction is performed, based on the detection results at the opposite ends, higher detection accuracy is required. Therefore, for example, when 2 micrometers are required as the detection accuracy, it is necessary to increase the sampling frequency to as high as 60 kilohertz.
Since the necessary sampling frequency is in proportion to the linear velocity and resolution, high processing speed capable of coping with the high-speed sampling becomes necessary for processing steps after the data sampling. Consequently, a cost required for color misalignment correction increases in substantial proportion to the increase in the speed of apparatus.
As detecting means for improvement in detection accuracy of pattern edge, a method of detecting the pattern edge by a charge coupled device (CCD) sensor having high accuracy and high resolution is proposed. However, even if such detecting means is used, problems such as complication and cost increase of the apparatus cannot be avoided.
For example, Japanese Patent No. 3254244 discloses the technology as follows. A toner image pattern is formed by superposing a second color toner image on a first color toner image and other toner image patterns are formed by shifting the relative position of the two color patterns by a predetermined amount. Each average density of the toner image patterns is detected by an optical sensor, and a misalignment between the first color and the second color and the direction of the misalignment are determined from output signals of the optical sensor to correct the misalignment.
In this technology, the detection of the misalignment is performed not by detecting the edge of a pattern image (line image), but by detecting an average output signal of the optical sensor based on the whole pattern. Therefore, it is possible to detect the misalignment at a sampling frequency as low as 500 hertz or below (for every 2 milliseconds), that is, about 1/100 times low as compared with the technology disclosed in Japanese Patent Kokoku No. Hei 7-19084.
Therefore, if a detection accuracy of the same level as that of the technology disclosed in Japanese Patent Kokoku No. Hei 7-19084 can be obtained by using the misalignment detection method disclosed in Japanese Patent No. 3254244, the hardware can be configured at a lower cost relating to the detection of misalignment, and therefore, considerable cost reduction can be obtained.
The technology similar to the misalignment detection method described in Japanese Patent No. 3254244 includes technologies disclosed, for example, in Japanese Patent Application Laid-Open No. Hei 10-329381, Japanese Patent Application Laid-Open No. 2000-81745, Japanese Patent Application Laid-Open No. 2001-209223, Japanese Patent Application Laid-Open No. 2002-40746, and Japanese Patent Application Laid-Open No. 2002-229280.
With regard to the technology of misalignment correction disclosed in Japanese Patent No. 3254244, if the maximum correction amount that has to be corrected is ±10 dots, then the misalignment correction amount and the direction thereof can be determined by forming 21 patterns obtained through shifting the relative position of the two colors dot by dot and reading extreme values of the patterns.
However, creating that many patterns increases not only wasteful toner consumption, but also the time required for automatic misalignment adjustment, which is not desirable.
Japanese Patent Application Laid-Open No. Hei 10-329381 discloses a method of detecting a misalignment more accurately, by calculating an intersection point of two lines when a reflected optical density is plotted on the y-axis with respect to a printing position parameter plotted on the x-axis.
In the method disclosed in Japanese Patent Application Laid-Open No. Hei 10-329381, even if the maximum correction amount is +10 dots, it is not necessary to form 21 patterns, and only 11 patterns may be formed through shifting by several dots appropriately, for example, by 2 dots. If the pattern is shifted by 5 dots, then only five patterns are required. Thus, highly accurate misalignment correction can be realized, while considerably reducing the number of patterns and the time required for misalignment adjustment.
Since misalignment adjustment is an operation that has nothing to do with the actual printing operation, if the processing time is too long, the time required for the first print increases accordingly. Therefore, the shorter time for the adjustment is better, considering the productivity.
However, there is a case where a positional deviation is obtained by calculating an intersection point of a linear approximate expression of two lines, and there is also a case where a deviation is obtained through arithmetic operation with a resolution having a shift amount of the line or less. In either of the cases, such an output characteristic as follows must be obtained. Specifically, a sensor output signal of each patch linearly increases or decreases with respect to a predetermined shift amount, that is, a line in which a determination coefficient R2 of each approximate expression of two lines is infinitely close to 1, must be obtained.
Therefore, by using a one-drum intermediate transfer belt type color image forming apparatus (writing density: 600 dpi) as illustrated in FIG. 33, it is verified whether the same degree of detection accuracy as that of the edge detection method disclosed in Japanese Patent Kokoku No. Hei 7-19084 is obtained through density detection of a two-color superposed pattern and calculation of the intersection point. As illustrated in FIG. 37, a patch is formed by superposing two color lines of black (Bk) as a reference color and another color (for example, cyan (C)), and a patch is also formed as one line as the minimum number of the lines obtained by superposing the two color lines. A detection pattern (alignment pattern) Pk for misalignment detection in the main scanning direction is obtained by continuously forming 13 patches (P1 to P13) with the relative position of the two colors shifted by an arbitrary amount. This detection pattern is read by a conventional optical sensor (alignment pattern detecting sensor) as illustrated in FIG. 38A, and output voltages of each of the patches with respect to the shift amount of the line other than the reference color are plotted to calculate an intersection point. Experiments were conducted on two references of 24 dots and 10 dots as references for line widths of two colors. The reason why the one-drum intermediate transfer type image forming apparatus was used is because it is desirable to keep an influence of the apparatus from a verified result as low as possible. The pattern used for verification was used for the pattern in the main scanning direction for the same reason as explained above.
As illustrated in FIG. 37, the respective patches are arranged along the scanning direction of the optical sensor, i.e., the direction of the transfer belt movement, and the color other than the reference color is shifted by an arbitrary amount in a direction orthogonal to the direction of the movement, in order to detect color misalignment in the main scanning direction.
As illustrated in FIG. 38A and FIG. 38B, the optical sensor includes a light emitting diode (LED) 700, a regular reflected light receiving element 701, and a diffused light receiving element 702, and these elements are supported by a support base 703. These elements are actually arranged in a substantially vertical plane with respect to a moving plane of an alignment pattern, but FIG. 38A illustrates the arrangement as a plan view obtained by rotating it by 90 degrees for simplicity. As illustrated in FIG. 38B, the support base 703 has a spot shape 700a of the LED 700, a spot shape 701a of the regular reflected light receiving element 701, and a spot shape 702a of the diffused light receiving element 702.
FIG. 39 illustrates the result of a case where the line width is 24 dots, and FIG. 40 illustrates the result of a case where the line width is 10 dots.
As illustrated in FIG. 39, in the case of the line width: 24 dots, in the approximate line obtained by plotted points on the negative side with respect to the extreme value, R2 is 0.9275. On the other hand, in the approximate line obtained by plotted points on the positive side with respect to the extreme value, R2 is 0.9555. Thus, the obtained output characteristic is not a straight line at all.
As a result of calculation of an intersection point based on the two approximate lines, a positional deviation of 34.74 micrometers (=0.82 dot) is obtained.
On the other hand, as illustrated in FIG. 40, in the case of the line width: 10 dots, in the approximate line obtained by plotted points on the negative side with respect to the extreme value, R2 is 0.9909. On the other hand, in the approximate line obtained by plotted points on the positive side with respect to the extreme value, R2 is 0.9985. Thus, the output characteristic quite close to a straight line is obtained.
As a result of calculation of an intersection point by the two approximate lines, a positional deviation of 12.91 micrometers (=0.30 dots) is obtained. The experiment conditions are as follows.    Detailed Parameters of the Detection Pattern as illustrated in FIG. 39:            Line width: 24 dots (=25.4/600×1000×24=1.016 millimeters)                    . . . commonly set for the Bk line and the color line                        Shift amount: 4 dots (=25.4/600×1000×4=169.3 micrometers)        Total number of patches: 13 patches (at P1 and P13, the two lines are not superposed perfectly, but at P7, the two lines are perfectly superposed on each other)        Repetition number of line: 1            Detailed Parameters of the Detection Pattern as illustrated in FIG. 40:            Line width: 10 dots (=25.4/600×1000×10=0.423 millimeters)                    . . . commonly set for the Bk line and the color line                        Shift amount: 1 dot (=25.4/600×1000×1=42.3 micrometers)        Total number of patches: 21 patches (at P1 and P21, the two lines are not superposed perfectly, but at P11, the two lines are perfectly superposed on each other)        Repetition number of line: 1            Detecting Sensor (detailed specification of the sensor as illustrated in FIG. 38A and FIG. 38B):    Light emission side:            Element: GaAs infrared light emitting diode (peak emission wavelength, λp=950 nanometers)        Top view type spot diameter: 1.0 millimeter            Light reception side:            Element: Si phototransistor (peak spectral sensitivity, λp=800 nanometers)        Top view type spot diameter:                    Regular reflected light receiving side: 1.0 millimeter            Diffused light receiving side: 3.0 millimeters                        Detection distance: 5 millimeters (distance from upper part of the sensor to a target surface (patch) to be detected)            Linear Velocity:            245 mm/sec [sampling frequency]        500 sampling/sec        
In the experiments, the alignment pattern was formed on the transfer belt so as to substantially match between the center of the Bk line and the center of the light receiving plane of the sensor.
As explained above, it is confirmed that the linearity as a basis of calculation of the intersection point is changed due to different line widths. This means that the detection accuracy of misalignment can be further improved by using a more appropriate method of forming the alignment pattern. In other words, this means that improvement in the detection accuracy of misalignment is achieved without cost increase.